1. Field of the Invention
This invention relates to a thermoplastic resin molded product having electrical conductivity on the surface thereof.
2. Description of the Related Art
Processes for making thermoplastics electroconductive include a process of blending an antistatic agent with thermoplastics or applying the agent on the surface of the thermoplastic molded product and a process of blending carbon black as an electroconductive material with thermoplastics. However, in the case of the former process, the surface resistivity of the resulting product is at most about 10.sup.9 .OMEGA./.quadrature. and yet there are drawbacks that the surface resistivity changes due to the ambient humidity; the antistatic effect is lost with lapse of time; etc. Further, in the case of the latter process, unless the more continuously the carbon black particles are present inside the thermoplastic resin sheet, the more the quantity thereof blended is increased, the aimed electrical conductivity cannot be obtained. However, if a large quantity of carbon black is blended, there are drawbacks that the mechanical strength of the molded product is notably reduced and the processability of the thermoplastic resin becomes inferior.
As electroconductive thermoplastic resin sheets solving the above-mentioned problems, an electroconductive thermoplastic resin sheet having a non-woven fabric consisting of electroconductive fibers and hot-melt-adhesive fibers (hereinafter referred to as electroconductive non-woven fabric) melt-adhered onto a thermoplastic resin sheet as a substrate is disclosed in Japanese patent application laid-open No. Sho 58-155917/1983, and an electroconductive thermoplastic resin sheet having a knitted or woven fabric consisting of electroconductive fibers and hot-melt-adhesive fibers melt-adhered onto and integrated with a thermoplastic resin sheet as a substrate is disclosed in Japanese patent application laid-open No. Sho 58-166035/1983.
These sheets are described therein to be subjected to heat forming such as vacuum forming, deep forming, etc. and used as molded products.
Usually, heat forming of thermoplastic resin sheets is in many cases carried out according to vacuum forming or pressure forming. According to these molding processes, a resin film in molten state is pressed onto a mold by air pressure to mold it. When the sheet disclosed in the above-mentioned Japanese Patent Application laid-open Nos. Sho 58-155917/1983 and Sho 58-166035/1983 is molded according to these processes, there is no particular problem if the surface of an electroconductive non-woven fabric or the surface of an electroconductive knitted or woven fabric is contacted with molds, but if the surface of the electroconductive non-woven fabric or that of the electroconductive knitted or woven fabric is not contacted with molds, electroconductive fibers protrude from the resin film to cause fluffing, and in addition, there is observed a phenomenon that electroconductive fibers are raised (or jut) from the resin film in the vicinity of the corner parts of the resulting molded products (this phenomenon hereinafter will be referred to as jutting phenomenon).
Further, press molding process is a process of molding by fitting a pair of male and female molds to each other, but in the case of this process, too, the more complicated the shape of the resulting molded products is, the more difficult it is to cause the clearance between the molds (i.e. a clearance formed when the male mold is fitted into the female mold in a state where no resin film is inserted thereinto) correspond to the thicknesses of the respective parts of the molded products; as a result, parts not contacted with the surfaces of the molds are necessarily formed. Thus, when an electroconductive thermoplastic resin sheet having an electroconductive non-woven fabric or an electroconductive knitted or woven fabric integrally melt-adhered onto both the surfaces thereof is subjected to press molding, then the electroconductive fibers in the electroconductive non-woven fabric or the electroconductive knitted or woven fabric inevitably cause partial fluffing or jutting phenomenon.
Furthermore, on the surface formed in contact with the molds is there sometimes appears to be no fluffing of the electroconductive fibers, but when the surface is rubbed by nails, cloth, etc., there is a problem that a portion of the electroconductive fibers peel off from the substrate to cause fluffing. The above-mentioned fluffing or jutting phenomenon not only deteriorates the appearance of the electroconductive molded products, but also when the surface is strongly rubbed, the electroconductive fibers fall off the surface of the molded products so that the environment is contaminated and further the electroconductive performance of the products is lowered; thus a serious obstacle to practical use of the products has been raised due to the above drawbacks.
When the surface of the electroconductive non-woven fabric or that of the electroconductive knitted or woven fabric is present on the surfaces of the molds, the electroconductive fibers are brought into a state where they are pressed onto the molds under the pressure exerted on the thermoplastic resin sheet and completely embedded inside the resin surface. Thus, no fluffing of the electroconductive fibers is observed.
Whereas, when the surface of the electroconductive non-woven fabric or that of the electroconductive knitted or woven fabric is not contacted with the surfaces of the molds, the thermoplastic resin sheet in a softened state is pressed onto the surfaces of the molds and the shape of the sheet is faithfully reproduced from those of the molds, but the electroconductive fibers in the electroconductive non-woven fabric or the electroconductive knitted or woven fabric cannot follow the deformation of the thermoplastic resin sheet and are hence protruded from the resin film. Since the deformation of the resin is particularly large in the vicinity of the corner parts of the molded products, the electroconductive fibers are completely separated from the resin to form a jutted state.
Further, even if no fluffing is observed when the thermoplastic resin sheet has been molded, fluffing often occurs when the surface of the resulting molded product is rubbed. This is presumed due to the fact that since the pressure under which the sheet is pressed onto the surfaces of the molds is so low that the electroconductive fibers have not been completely embedded in the resin film (namely the electroconductive fibers have not been completely coated by the resin layer or the layer is very thin and also liable to be peeled off).
The present inventors have made extensive research in order to overcome the above-mentioned problems raised when an electroconductive thermoplastic resin sheet obtained by having an electroconductive non-woven fabric or an electroconductive knitted or woven fabric integrally melt-adhered onto at least one side surface of a thermoplastic resin film is subjected to vacuum, pressure or press forming. As a result, we have found that when a crosslinked and cured coating composed mainly of an unsaturated resin and a reactive diluent and having a coating thickness of 1 to 15 .mu.m, preferably 1 to 10 .mu.m is formed on the surface layer of the electroconductive thermoplastic resin sheet, followed by molding the resulting sheet in a pair of male and female molds at least one of which has a heat-resistant rubber on the surface layer thereof, then there is obtained a molded product which makes it possible to prevent fluffing and jutting phenomenon of the electroconductive fibers without lowering electrical conductivity and further even when strongly electroconductive fibers, and have achieved the present invention.